Ink jet printing processes are essentially of two types:
In continuous stream ink jet printing systems, ink is emitted in a continuous stream under pressure through a nozzle. The stream breaks up into individual droplets at a certain distance from the nozzle. If a specific location on the recording sheet has to be printed the individual droplets are directed to the recording sheet, otherwise they are directed to a collecting vessel. This is done for example by charging unnecessary droplets in accordance with digital data signals and passing them through an electrostatic field which adjusts the trajectory of these droplets in order to direct them to the collecting vessel. The inverse procedure may also be used wherein uncharged droplets end up in the collecting vessel.
In the non-continuous process, or the so-called “drop-on-demand” systems, droplets are generated in accordance with digital data signals only if a specific location on the recording sheet has to be printed.
The speed of ink jet printers of the newest generation is ever increasing for economic reasons. Recording sheets particularly suitable for these printers need to absorb the inks very rapidly. Recording sheets particularly suitable for this purpose comprise nanoporous inorganic compounds such as oxides or oxide/hydroxides.
Images produced in this way need to have excellent storage stability even under adverse climatic conditions. This can only be achieved by using a finely tuned system of inks (respectively the dyes contained therein) adapted to these nanoporous recording sheets.
Such recording sheets available today do not have all the properties required when printed with inks available today. In particular, the storage stability of images printed onto these recording sheets has to be improved. These images are not particularly stable even in the dark when they are in contact with ambient air, which normally contains impurities such as ozone or nitrogen oxides. The images are strongly altered or even destroyed in a short time when they are in contact with ambient air. These phenomena are described for example in “Ozone Problem with Epson Photo Paper”, Hardcopy Supplies Journal 6 (7), 35–36 (2000).
Although quite a number of different copper complex monoazo dyes with magenta or red hue have already been proposed as dyes for ink jet printing, none meets all the necessary requirements. In particular, the storage stability in the dark of images containing these dyes when they are in contact with ambient air containing impurities is not sufficient.
The copper complex monoazo dye of formula (I) is described in patent application EP 1,086,999 (Example No. 19).

The copper complex monoazo dyes of formulas (II) and (III) are described in patent application JP 2000-265,099.

The copper complex monoazo dye of formula (IV) is described in patent application JP 63-30,573.

All these copper complex monoazo dyes, representing the state of the art, do not satisfy all the required demands if they are used in the formulation of inks for ink jet printing that should provide magenta colorings on nanoporous recording sheets with particularly high brilliance and, at the same time, with improved storage stability in the dark of images containing these dyes when they are in contact with ambient air containing impurities. Furthermore, the diffusion fastness of these dyes under conditions of high humidity and temperature is not sufficient.
Dyes used for the preparation of such inks need to have a good solubility in the essentially aqueous ink liquid, they have to penetrate into the recording sheet and should not show dye aggregation on the surface of the recording sheet (“bronzing”). They need to provide printed images having high optical density, good water fastness, good light stability and good storage stability even under adverse conditions. They need to be stable in the ink even when the ink is stored for a long time under adverse conditions.
Various types of ink compositions have been proposed. Typical inks comprise one or more dyes or pigments, water, organic cosolvents and other additives.
The inks have to satisfy the following criteria:    (1) The ink gives images of excellent quality on any type of recording sheet.    (2) The ink gives printed images exhibiting good water fastness.    (3) The ink gives printed images exhibiting good light stability.    (4) The ink gives printed images exhibiting excellent smudge behavior.    (5) The ink gives printed images exhibiting excellent storage stability under conditions of high temperature and humidity.    (6) The ink does not clog jetting nozzles of the ink jet printers even when these are kept uncapped while recording is suspended for long periods.    (7) The ink may be stored for long periods of time without deterioration of its quality.    (8) The values of the physical properties of the inks, such as viscosity, conductivity and surface tension are all within defined ranges well suited for the intended use.    (9) The ink has to be non-toxic, non-flammable and safe.